Optical fiber is produced by drawing the fiber from a preform. The refractive index profile of the drawn fiber is substantially the same as the profile of the preform from which it is drawn. Deviations from the desired refractive index existing in the preform will thus be repeated in the fiber and may result in unacceptable transmission characteristics. It is therefore prudent to profile the refractive index of the preform before drawing fiber from it to avoid wasting time and money producing worthless scrap.
Historically, refractive index profile measurements have been relative index measurements; the refractive index of the fiber core relative to the doped or undoped silica cladding. It is this relative difference which gives an optical fiber its light guiding properties.
As currently practiced, the manufacture of single mode preforms comprising a core and a clad by the "modified chemical vapor deposition" (MCVD) process has associated cost penalties. This process requires many depressed cladding layers to be built up before the core layers are deposited. These depressed cladding layers are needed to obtain the proper core-to-clad ratio and acceptable loss.
A more economical alternative preform manufacturing technique uses high purity fluorinated silica tubes as starting substrates for MCVD together with rod-in-tube technology as described in U.S. patent application, Ser. No. 099,441, Continuation under Rule 60 of Ser. No. 856,739, filed on Sept. 23, 1987 in the names of J. W. Baumgart et al and assigned to AT & T Technologies, Inc., which is now U.S. Pat. No. 4,820,322 issued on Apr. 11, 1989. By starting with fluorinated tubes the deposition of many fewer depressed cladding layers are required, rendering MCVD more competitive.
The above alternative, as well as others that are being used such as the "outside vapor deposition" (OVD) process, could present profiling problems that current generation preform profilers would not be able to handle.
In the OVD process, the glass precursor vapor is introduced into a hydrolyzing flame and particulate material is formed. This material emanating from the flame is directed toward a mandril on which it is deposited. Following such deposition, the deposited material is consolidated into a transparent glass, the mandril removed and the resultant hollow tube collapsed to form a solid, cylindrical optical fiber preform which may not have a silica cladding.
The current method and apparatus for nondestructively determining the refractive index profile of an optical fiber preform have been disclosed in U.S. Pat. No. 4,227,806 granted to Lawrence S. Watkins (incorporated by reference herein). This approach works well with a preform having a core and a silica clad. However, prior art apparatus is not capable of accurately profiling the refractive index of an unclad preform, or component of a preform relative to silica or another reference.
Prior art apparatus is shown schematically in FIG. 1. Tank 10 has heads 11 and 12 supporting side windows 13 and 14. A preform 15 having a core 16 and cladding 17 is suspended in a fluid such as index matching oil 18 with its longitudinal axis parallel to the plane of windows 13 and 14 by a support (not shown).
Oil 18 with its index of refraction close to that of cladding 17 is usually used instead of air when accuracy is required to avoid unpredictable variations in the refraction of beam 19 caused by non-circularity of the surface of cladding 17.
In operation, a narrow monochromatic beam of light 19 from light source 20, focused at the center of core 16, is scanned across preform 15 such that beam 19, as it scans, is always substantially perpendicular to a plane containing the longitudinal axis of preform 15. The refraction angle of beam 19 exiting preform 15 is detected by detector 21. A computer 22 programmed to process the detected data constructs the refractive index profile and displays it by employing an output device such as plotter 23.
In the case of a fluorinated silica tube to be used as a substrate for MCVD, there is a need to profile the down-doped tubes as part of an initial quality check. Again, however, there is no silica reference without which the amount of fluorine down-doping could not be reliably measured. A silica jacket could be collapsed over an unclad object, such as a fluorinated tube or an OVD preform, in order to accurately measure its index depression relative to silica. This, of course, would be a time-consuming, inefficient and wasteful test procedure. Destructive tests, such as slicing and direct analysis could also be employed with obvious disadvantages.
It was also attempted to use index matching oil 18 as a surrogate for the silica cladding, but for reasons that will be discussed hereinafter, this procedure does not provide accurate results.
Thus, there is a need for an efficient, accurate and nondestructive method and apparatus for constructing, relative to silica, the refractive index profiles of unclad preforms, unclad components of preforms and other unclad transparent bodies.